1. Field of the Invention
The present invention relates to a fixing device, which grips a recording medium on which a toner image has been formed between a pair of mutually opposed fixing members, and fixes the toner image to the recording medium by applying heat and/or pressure, and an image-forming apparatus comprising this fixing device.
2. Description of the Related Art
In an image-forming apparatus that utilizes a digital photography system, an electrostatic latent image is formed on the surface of a photosensitive drum, which is one example of an image carrier, this electrostatic latent image is made into a visible image by using a toner or the like, which is a developer, to develop the electrostatic image on the photosensitive drum, the developed image is transferred to a transfer material by a transfer device, the transfer material carries the image, and a fixing device fixes the toner image onto the transfer material using pressure and heat. This image-fixed transfer material passes through a paper discharge path and is discharged outside the apparatus. The fixing device grips the recording medium between a pair of opposing fixing members, for example, either rotating members such as rollers or belts, or fixed members such as pressure pads, or a combination thereof, and applies heat and/or pressure to fix the toner image onto the recording medium.
For example, a fixing roller equipped with a built in heater, which is heating means, and a pressure-applying lever, such as a pressure roller that makes contact with the fixing roller are arranged as the fixing members. A toner transfer agent-bearing recording medium that has reached the fixing device is introduced into a fixing nip, which constitutes the contact part of the fixing roller and pressure roller. In the process of the recording medium passing through the fixing nip, the transferred toner image is fixed to the surface of the recording medium by being subjected to heat and pressure.
However, when a paper jam creates a state in which the recording medium becomes stuck in the fixing nip of this fixing device and the apparatus stops, jam processing, in which the user removes this recording medium, is extremely difficult due to the fact that the recording medium is stuck in a state in which pressure is being applied to the fixing nip. For this reason, a method in which a manual release lever is provided in the fixing device prior to the user carrying out jam processing to release the pressure at the time of jam processing has been well known for some time now.
The problems with the manual method are the complicated nature of the lever operation and forgetting to operate the lever. Therefore, Japanese Patent Application Laid-open No. H5-173446 (hereinafter referred to as Prior Art 1) proposes an automatic release device, which uses a cam device to automatically release the contact pressure of the fixing nip.
Japanese Patent Application Laid-open No. 2001-318555 (hereinafter referred to as Prior Art 2) proposes a configuration which causes a cam attached to a release lever to act on a direct-pressure-applying lever to release the pressure-applying state of the fixing members.
Japanese Patent Application Laid-open No. 2000-214718 (hereafter referred to as Prior Art 3) proposes a configuration in which releasing the pressure-applying state of the fixing members is linked to the opening and closing of a cover.
The above-mentioned Prior Art 1 comprises a cam device for automatically releasing the contact pressure of the fixing nip, but this requires the disposition of a new drive mechanism such as a dedicated motor, increasing the weight and cost of the device. In recent years, it has been desirable to make image-forming apparatuses smaller and faster, and making the fixing rollers and belts smaller in diameter has resulted in shorter nips for holding the recording medium. On the other hand, since the speed at which the paper passes through has become faster, the heat applied to the recording medium at fixing is no longer sufficient, making it difficult to achieve satisfactory fixing performance. For this reason, a stronger pressure-applying force is needed than in conventional devices so as to ensure a bigger nip. The problem with this is that the actuating force on the conventional manual release lever becomes too great, making it difficult for the user to operate.
Further, in the case of a configuration which causes a cam attached to a release lever to act on a direct-pressure-applying lever to release the pressure-applying state of the fixing members as disclosed in the above-mentioned Prior Art 2, the problem is that strengthening the pressure-applying force increases the frictional force between the cam and the direct-pressure-applying lever, thereby lowering the sliding performance of the sliding contact surface, and also increasing the wear on the sliding contact surface.
Even when the release of the pressure-applying state of the fixing members is linked to the opening and closing of a cover as disclosed in the above-mentioned Prior Art 3, the problem is that strengthening the pressure-applying force increases the frictional force of the contact surfaces of the release lever and opening-and-closing cover, lowering the sliding performance of the sliding contact surfaces and increasing the mutual wear on the sliding contact surfaces.
To solve for problems such as those mentioned above, Japanese Patent Application Laid-open No. 2006-48005 (hereinafter referred to as Prior Art 4), which discloses a fixing device that features good operability and excellent durability while producing favorable pressure-applying force, is known. In the configuration of this Prior Art 4, a fixing device, which fixes a toner image onto a recording medium by gripping the recording medium between a pair of opposing fixing members and applying heat and/or pressure, has a pressure-applying lever for causing the one fixing member to apply pressure to the other fixing member; a pressure-releasing lever, which is rotatably supported on the pressure-applying lever; a locking member, one end of which is rotatably attached to the pressure-releasing lever; and an elastic member, which is latched to the other end of the locking member, and pulls the locking member in the locking direction, and is characterized in that the rotation of the pressure-releasing lever displaces the location of the locking member, enabling a pressure-applying position, in which the one fixing member presses against the other fixing member via the pressure-applying lever, to switch over to a pressure-releasing position, in which the one fixing member moves away from the other fixing member, and in the pressure-applying position, the pressure-applying lever and the locking member, which are fastened together at a location at the opposite end of the rotating central shaft of the pressure-applying lever, are pulled in the locking direction by the elastic member.
In accordance with this configuration, a pressure-releasing lever is rotatably supported on a pressure-applying lever that causes the one fixing member to apply pressure to the other fixing member, one end of a locking member is rotatably attached to this pressure-releasing lever, and an elastic member, which pulls the locking member in the locking direction, is latched to the other end of the locking member, and when the pressure-releasing lever rotates, the location of the locking member is displaced, and a pressure-applying position, in which the one fixing member presses against the other fixing member via the pressure-applying lever, is switched over to a pressure-releasing position, in which the one fixing member moves away from the other fixing member. Thus, this configuration does not have a cam or driving means for driving the cam as in the past, and can be made lightweight and inexpensive. Since the rotation of the pressure-releasing lever also displaces the location of the locking member to which the elastic member is latched to one end, elastic means extension is minimal and the actuating force at operation is small, making user operation easy. Since the pressure-applying lever and the locking member are fastened together at the furthest location from the rotating central shaft of the pressure-applying lever, and the elastic member pulls thereon, the part that slides over the locking member can be subjected to minimal frictional force, achieving high durability.
However, in recent years, the glossiness required for an output image has come to differ by the application and type of paper of the recording medium, and fixability for a variety of recording media is required.